The demand for communication systems with higher capacities has pushed the common design approaches of wavelength-division-multiplexed (WDM) systems to their limits. A typical configuration of a point-to-point WDM system includes a number of optical transmitters, an optical multiplexer, spans of transmission fiber, optical amplifiers (usually erbium-doped fiber amplifiers, EDFAs), dispersion compensating devices, an optical demultiplexer and a number of optical receivers. Unfortunately, the usable gain bandwidth for the optical amplifiers currently used, for example the EDFAs, is limited and not very broad, and the distortion of the signal does not allow for transmission over very long optical transmission links. This has led to the investigation of alternate methods for amplification with greater broadband capabilities that allow for longer spacing in-between amplification and longer transmission distances.
The use of Raman amplification has been proposed and demonstrated for compensating losses in all-optical transmission systems. Raman amplification is achieved by launching high-power pump waves into a silica fiber at a wavelength lower than the signal to be amplified. Amplification occurs when the pump wavelength gives up its energy to create new photons at the signal wavelength. Since there is a wide range of vibrational states above the ground state, a broad range of transitions may provide gain, of which, typically, 48 nm is usable gain. Raman gain increases almost linearly within the wavelength offset between the pump wavelength and the signal wavelength, peaking at a distance of typically 100 nm and then dropping off rapidly with increased offset. Ultra-broad Raman gain bandwidth can be achieved by combining the Raman amplification effect of multiple pump waves selected carefully for the wavelength domain. See, for example, H. Kidorf, K. Rottwitt, M. Nissov, M. Ma, and E. Rabarijaona, “Pump interactions in a 100-nm bandwidth Raman amplifier,” IEEE Photonics Tech. Lett. 11, 530, 1999. Additionally, the positions of the gain bandwidth within the wavelength domain of each pump can be adjusted by tuning the pump wavelength. Compared to commonly used erbium-doped fiber amplifiers (EDFAs), Raman amplifiers exhibit several fundamental advantages such as low noise, fixed gain profiles which are independent of signal and pump levels; they are also operable in a plurality of signal bands since Raman gain peak changes with pump wavelength.
Despite all of its advantages, there are some degradation effects related to Raman-pumped WDM systems. For example, when using a plurality of Raman pumps with varied wavelengths as a collective Raman pump to pump an amplification fiber, the different pump wavelengths of the collective Raman pump generate gain curves having different gain maxima. These gain curves, when combined, thereby create an uneven gain profile. This uneven gain profile is referred to as containing gain ripple. After concatenations of several Raman amplification spans, the gain ripple can accumulate and ultimately limit system performance. In addition, power fluctuations in time within the plurality of Raman pumps, which is so often the case, may also lead to amplified fluctuations and gain ripple, which also degrades system performance.
In current Raman-pumped systems, to correct for gain ripple from span to span, dynamic gain equalizing filters (DGEFs) are implemented in each span to increase the gain flatness. Unfortunately, DGEFs are costly devices and introduce excess loss that needs to be compensated for by adding optical amplifiers to each span further increasing the cost and complexity of a system.